In the construction of buildings, one of the most common elements is gypsum wallboard, often known as "drywall", used to construct the walls and/or ceilings. Walls made from gypsum wallboard are conventionally constructed by fixing the panels to studs, and filling and coating the joints with a specially prepared adhesive called a joint compound. This process generally proceeds in the following fashion: a taping grade joint compound is placed within the joint formed by the abutted edges of the wallboards, and a perforated tape is embedded within the taping compound. When dry (or set), a second, topping grade joint compound is coated over the joint. This may be sanded lightly, and third coat is applied and conventionally finished. Another grade of joint compound is an all-purpose grade which may be used, as the name suggests, for embedding the tape and for the finishing coats. In some instances, a patterned effect is given to the all-purpose joint compound to leave a textured finish.
The primary difference in the past between the various grades of joint compound has been in the differences in the amounts of each ingredient. Thus, no matter what the grade, joint compounds invariably include a filler and a binder. More binder is used for the taping grade than for the topping grade. Typical fillers may be calcium carbonate, calcium sulfate hemihydrate, or calcium sulfate dihydrate. As is obvious, the choice determines whether the joint compound hardens by drying or by setting. An example of a setting type joint compound is taught in U.S. Pat. No. 3,297,601.
In addition, conventionally it has been necessary to include asbestos as a key ingredient. However, asbestos is becoming more and more an unacceptable health hazard, particularly where airborne. Health experts have indicated that not only does it cause mesothelioma, it may cause other forms of lung cancer as well, and may be a greater source for the latter than automobile fumes. Because of such effects, the Occupational Safety and Health Administration has established a U.S. Asbestos Standard, published in vol. 37, pp. 11320-11322 of the Federal Register, which provides that, at no time may the concentration of the asbestos fibers longer than 5 micrometers exceed 10 in number per cc. of air. In addition, effective July 1, 1976, the airborne concentration of asbestos longer than five micrometers in any working area, on the average, may not exceed two fibers per cc. of air. The asbestos in joint compounds becomes airborne in two ways: when the compound is dumped into water, and when the dried or set coat is sanded prior to the application of another coat or some other finish. Hand mixing the dry mix of ingredients into water is a primary first step, as many joint compounds are packaged dry and wetted by the applicator on site. The microscopic fineness of the asbestos causes it to literally puff into the air into an invisible cloud. Actual measurements have indicated that such mixing may cause a concentration of asbestos fibers longer than five micrometers of up to 55 fibers per cubic centimeter of air in the breathing area in the vicinity of the mixing period. Sanding may cause a fiber concentration of slightly over 3 fibers per cc. of air over an extended period of time. Clearly, by 1976, the conventional formula and the use of asbestos therein will hardly meet the above Asbestos Standard.
Thus, an asbestos-free joint compound is needed. Heretofore, such has not been readily obtainable, due to the unique combination of features provided by asbestos, especially concerning the workability of the joint compound.